Dual-port semiconductor memories, or video random access memories (VRAMs), employ some arrangement of serial access memory for storing a row or a part of a row of data separate from the matrix of storage cells, which forms the main memory array. Stages of the serial access memory, or the serial register, may be either static or dynamic circuits. Static circuits are preferred because they do not lose their contents during any extended inactive period. Typically the static circuit is a latch circuit.
For a serial readout operation, data is transferred from a row of the memory array by way of the bitlines to the serial register stages. This read transfer operation is accomplished by opening transfer gates between pairs of complementary bitlines of the memory array and the complementary storage nodes of the serial register stages, or latches. The states of the serial register stages are statically held and must be switched by the signals applied from the bitlines through the transfer gates. Sometimes it is difficult to switch the states of the serial register stages because the bitline signals are weak and the complementary states of those signals may not be fully separated by the time the transfer to the serial register stages is to take place.
To facilitate the switching of the stages of the serial register, some special designs have been utilized. One such special design uses a virtual power supply for the stages of the serial register. The virtual power supply is switched off when the stages of the serial register are to be loaded with new data. By temporarily switching off the power, the latching signals are diminished so that new data signals on the pairs of complementary bitlines can readily overwrite the states previously stored in the stages of the serial register. Because of high capacitance related to the virtual power supply nodes and because of auto-biasing within the stages of the serial register, the magnitudes of the bitline signals still may be insufficient to assure error free overwrite by the new data read out of the memory array.